docs: update readthedocs document loader options (#29556)
Hi there!
This PR updates the documentation according to the code.
If we run the example as is, then it would result in the following
error:
It seems that this part of the code already supplied the required
argument to the BeautifulSoup4:
https://github.com/langchain-ai/langchain/blob/0c782ee5479f1eadf6cc532d7f27617bd2671fe9/libs/community/langchain_community/document_loaders/readthedocs.py#L87-L90
Since the example can only work by removing this argument, it also seems
legit to remove it from the documentation.
